1. Field of the Invention
The present invention generally relates to electrochemical cell design and fabrication and, more particularly, to techniques for forming electrically insulating separator elements between two electrically conductive surfaces.
2. Description of the Related Art
In microelectronics technology, the continuing trend of ultra large scale integration is driving the electronics industry to explore new materials and processes for fabricating devices having smaller size and better qualities. This is particularly relevant to the manufacture of electrochemical power supplies of high energy and power density for portable electronic devices such as portable computers, telephones, audio, video devices, pacemakers, etc.
Increasing power per unit volume and improving discharge characteristics depends on the ability to fabricate thinner electrochemical cells, of which thinner separator layers are a necessary ingredient. As such, the integrity and reliability of thin separators (&lt;50.mu.) are of great importance and, consequently, have received increasing attention in the last decade.
The separator within an electrochemical cell has the primary function of physically and electrically separating the anode from the cathode thus preventing a short circuit within the cell. The separator may be an electrical insulator which permits ions to flow between the anode and the cathode. Typically, separators consist of mats, pellets, papers, cloths or polymer sheets that contain an electrolyte within their pore structures. The ionic conduction occurs through the electrolyte phase contained in the contiguous pores of the separator when the separator is placed in intimate contact with the anode and cathode and the cell is charged or discharged. Therefore, the separator should preferably be dimensionally stable and porous, with high electrolyte absorption and retention and a low ionic resistance.
The separator should preferably also be flexible. It should accommodate electrochemical expansion and contraction of the anode and cathode during discharge and/or charge, and it should accommodate thermal expansion and contraction of the cell. The separator should also preferably be resistant to oxidation and reduction. It should preferably be insoluble in the electrolyte and it should resist corrosion by the other components of the cell and by the reaction products that are generated within the cell. The physical and chemical properties of the separator should be compatible with the equipment and processes which are used to manufacture the electrochemical cell. The separator should preferably be thermally stable during the manufacture, storage, and use of the electrochemical cell. Finally, to minimize the ionic resistance of the separator, the separator should be made as thin as practical in consideration of the manufacturing process and the desired performance of the electrochemical cell.
In the prior art, these characteristics have been achieved in part by the use of silicon oxide compounds such as fumed silica, silica gel, silica aerogel, silica xerogel, silica hydrogel, silica sol, and colloidal silica. Other materials such as alumina, magnesia, kaolin, talc, diatomaceous earth, calcium silicate, aluminum silicate, calcium carbonate, and magnesium carbonate have also been used as inert fillers, extenders, or pore formers in separators.
In the prior art, various applications of these materials have led to the development of various types of electrochemical cell separator/electrolyte structures. In general, such prior art structures have been constructed in the form of an ionically conductive gel, a pellet, an ionically conductive solid or a liquid electrolyte filled porous separator element.
In one prior art application, for example, fumed silica is used as a gelling agent for hydrochloric acid electrolytes. The silica-hydrochloric acid gel can be impregnated into a polymeric foam to form an ion selective separator membrane for redox cells where the transport of chloride ions is desired. Similarly, in another application, silica gel is used to immobilize sulfuric acid electrolytes and thereby provide a method for making hermetically sealed dry accumulators.
In thermal battery technology, for example, a rather different approach is taken to form the separator element. Here, the separator element is fabricated by pressing blended and fused mixtures of fumed silica and an electrolyte salt into a pellet. Such prepared pellet separators are subsequently placed between the cathode and the anode electrodes of the thermal battery cell. With the application of heat, the electrolyte salt in the pellet becomes molten and ionically conductive, and thereby activates the battery cell. At elevated temperatures, the thermal battery separator element is held together by the capillary forces of the molten salt in the porous fumed silica matrix and does not require any separate binding material to retain the shape of the separator.
However, this technology has not been successfully applied to the manufacturing of ambient temperature battery cells. This is due to the weak capillary forces of most ambient temperature battery electrolytes which are not sufficient to maintain the integrity of the unbounded silica-electrolyte structure. Therefore, the separators for ambient temperature applications are thick, and provide a limited range of size and shape for design considerations.
Further, in thin film battery technologies, salinized fumed silica or alumina are used as inorganic filler materials to manufacture solid electrolytes. Such electrolyte films are formulated with a combination of a polymer matrix material, an electrolyte salt, a compatible solvent (plasticizer), a crosslinking agent, and a finely divided filler material. The resulting structure is a flexible and self-supporting polymer electrolyte film without porosity. In this film, the conduction of ions occurs in solid state within the electrolyte plasticized polymer. Particularly, the addition of inorganic fillers enhances the physical strength of the electrolyte film and substantially increases the subsequent level of electrolyte absorption, and subsequent substitution of the plasticizer. As a consequence, a separator is not required and the solid electrolyte serves the purpose of separating the anode from the cathode.
In thin film battery technology, the solid electrolyte films can be readily printed onto a surface of the electrode elements using screen printing and stencil printing processes. However, in the absence of a distinct separator element, compression of the electrolyte leads to short circuits and/or displacement of the electrode material. In fact, this condition is improved by the use of screen printed or stencil printed "stud" elements or "standoff" elements to strengthen the electrode elements and solid electrolyte elements against compression. Nevertheless, the low ionic conductivity of the solid electrolytes severely limits the use of these thin film batteries, particularly when high rate charge and/or discharge is required. Similarly, "studs" or "standoff" elements can be screen printed or stencil printed onto the surface of electrode elements in electrochemical capacitors. In such applications, the standoff elements are made of silica reinforced epoxy polymers to increase their strength.
However, in the prior art, much effort has been devoted to the production of silica incorporated microporous separator structures which use liquid electrolytes. Examples of such structures may include microporous silica separators from organosilicon polymers and composite separators comprising a silica filler material and a polymer binder.
In secondary battery technologies, for example, an ultrathin layer of microporous silica separator material can be formed on a lithiated ternary transition metal oxide electrode by decomposition of an organosilicon polymer solution. During the manufacturing process, a thin layer of an organosilicon polymer solution is coated on the surface of the battery electrode. After drying to remove the solvent, the coating is cured to a glassy film and then subjected to plasma oxidation to form micropores within the film. The resulting ridged, fenestrated silicate film serves as the separator. However, precautions must be taken to prevent unwanted oxidation of the active electrode by the plasma.
In the prior art, many of the aforementioned silica or non-silica filler materials are used to fabricate microporous battery separators having a composite structure. These filler materials are produced as finely divided solid particulates, and used as a vehicle for introducing porosity into the microporous separator and for reinforcing the polymeric binder material utilized to fabricate the separator. In fact, this composite nature of the separator element renders separators with high strength and flexibility. During an exemplary manufacturing process, the solid particulate material is blended with a binder material of choice and then this blend is loaded with a suitable solvent to form a paste. The separator element is formed by extruding this paste into a sheet form, and subsequently curing this sheet material to remove solvent and impart porosity to the separator structure.
In an early prior art application, silica gel was used as an inorganic filler and extender for such microporous separators comprised of high molecular weight polyolefin binders. Similarly, in another application, precipitated amorphous silica was used to manufacture microporous polymeric battery separators. In these separators, the silica was used in small proportions (typically &lt;30%) to reinforce the polymeric material and to introduce porosity.
However, yet another prior art application includes a battery separator consisting of a polymer and silica filler in which the silica filler comprises up to 97% of the composite matrix. In this application, the preferred filler is precipitated silica or fumed silica.
Unfortunately, such high filler content adversely affects the mechanical properties of the separator and lowers its strength and flexibility. In order to overcome these limitations, in the prior art, such composite separator elements were extruded and laminated onto both sides of a fibrous polymeric sheet to improve the strength and flexibility of the separator element, i.e., to make it self supporting.
Although such prior art processes yield functional composite separators, these processes provide relatively thick separator layers which can cause excessive separation between the electrodes and thus increase the overall resistance of the separator.
Additionally, material handling during the manufacturing process presents serious problems due to very small product dimensions which increase the production cost and labor. Similarly, lack of a conformal bonding between the electrodes and the separator element prevents the precise and secure positioning of the separator element in the electrochemical cell structure and wastes valuable cell volume. Furthermore, such inefficient packaging of the electrochemical cell increases the distance between the electrodes, thus further lowering the efficiency of the cell.
Thus, silicas and polymers have been used to make separators for electrochemical power supplies; and printing processes have been utilized to make solid electrolytes for printed electrochemical power supplies and microprotrusion separators for electrical charged storage devices.
In a patent issued to the assignee of the present invention, U.S. Pat. No. 5,882,721, a separator was disclosed which is formed by printing a thin layer of a separator precursor solution on the surface of one of the electrochemical cell electrodes, and curing the thin layer of separator precursor solution so that it transforms into a microporous composite separator element on the electrode. That patent application is hereby incorporated by reference, in its entirety, in this application.
Specifically, in that application, a separator precursor solution is formulated as an ink comprised of a solid particulate material dispersed in a solution of a polymer binder which is dissolved in a suitable solvent. In the preferred embodiment the solid particulate material is silica aerogel material which constitutes the major component of the printed separator. In the separator precursor solution, the ratio of the binder to the solid particulate material is selected between 5/95 and 50/50. The polymer binder of the ink solution may preferably comprise Ethylene-Propylene-Hexadiene Monomer (EPDM). The ink solution also comprises 1,2,3,4-Tetrahydronaphthalene (THN) solvent to dissolve Ethylene Propylene Hexadiene Monomer (EPDM). The solvent content of the ink solution ranges from 60% to 95% by mass.
Hence, in that application, a separator body is formed on an electrode plate comprising intermeshed networks of pore spaces between a solid particulates bonded together by a polymer binder. That process results in separator structures in the form of a porous composite layer on an electrode of an electrochemical cell. These structures are non-self supporting, ultra-thin, flexible and conformally bonded to the underlying electrode surface.
That application also discloses the use of that separator in an electrochemical cell which includes, in addition to the layer of porous composite separator material disposed on the first electrode, a second layer of electrode material superjacent to the porous composite layer and a liquid electrolyte. The liquid electrolyte is contained within the plurality of pores of the porous composite separator material so that the electrolyte provides a conducting medium between the first and second electrode layers.
Ethylene Propylene Hexadiene Monomer (EPDM) is an elastomer, and electrodes formulated with EPDM are advantageously calendared under high pressure to assure good particle-to-particle contact. It would be advantageous to form a separator using a binder which does not require pressure during fabrication to achieve high performance.
In addition, EPDM may be electrochemically unstable at high positive potentials, and is used as a binder in anodes, but not cathodes. It would be advantageous to form an electrochemical cell having a separator formed with a binder which is common to the binder in both the anode and cathode.